1. Field of Invention
The present invention concerns a process for the surface treatment of a roll for a rolling mill. It relates more particularly to a process for making the surface of a cold-rolling-mill roll for the purpose of imparting controlled roughness to a steel sheet.
2. Description of Prior Art
Various prior proposals of the Assignees of the Applicant, notably their Belgian Pat. No. 870 609, have disclosed a method of marking the surface of a rolling mill roll by means of an intermittent laser beam. This known method enables the formation, in this surface, of micro-cavities or micro-craters which are distributed according to a predetermined pattern and which, being imprinted in the surface of the sheets, impart to the latter the required roughness. When the roll have been used for a certain length of time, they become less rough, and have to be reconditioned in order to recover their original surface condition. A worn roll has to be dismantled from the mill, corrected, marked again, then re-installed and adjustments involved mean loss of time and of substantial expense, which are reflected in the cost of the roll. It is therefore advantageous to space out these reconditionings, i.e. to reduce the rate at which the roughness wears out.
Belgian Patent Application No. BE-A-901 055 discloses a method of improving the life span of mill rolls marked by the above-mentioned method. This method consists in applying a metal coating to the surface of the roll, then treating the coated roll by means of an intermittent laser beam in such a way as to locally fuse the metal coating and the underlying roll surface and thus to instigate the formation of alloyed beads or rims around the micro-craters. By this method one can perceptibly improve the hardness and therefore the life span of the actual beads. It has, however, become evident that the alloyed beads do not always adhere sufficiently to the metal coating and that beads sometimes become torn off during rolling. If this is the case, a bead can be torn off even when the roll has only been used for a short time. The life span of the roll is therefore reduced, and moreover the bead thus detached and carried along can cause deterioration of the surface of the rolled product.